A man rows downstream at 20 km/hr and rows upstream at 15 km/hr. At what speed he can row in still water?

17.5 Km/hr

Let,

Speed of boat in still water= x

Speed of stream/current = y

According to the question:

Upstream speed (Us) = 15 km/h i.e. x - y = 15 km/h    ........(i)

Downstream speed (Ds) = 20 km/h i.e. x + y = 20 km/h   ........(ii)

Speed he can row in still water = x

Adding (i) & (ii):

x - y + x + y = 15 +20

2x = 35

x = 17.5 km/hr